1. Field of the Invention
The present invention relates generally to an electrically controlled brake system for a vehicle, and more particularly to an arrangement capable of suitably controlling an amount of brake applied to a wheel of the vehicle, irrespective of whether the vehicle is running in the forward direction or in the backward direction, and/or irrespective of whether the vehicle is in a stop state or not.
1. Discussion of the Prior Art
As a device for decelerating or stopping a motor vehicle, there is known a hydraulically operated brake, in which a friction member is forced against a rotor rotating with a vehicle wheel, by a wheel brake cylinder which is activated by a pressurized fluid produced by a master cylinder when a brake pedal or other brake operating member is operated. In recent years, there has been proposed an electrically controlled brake system wherein the operating force or stroke or other operating amount of the brake operating member is electrically detected, so that the amount of brake applied to the wheel is controlled so as to provide a braking effect which corresponds to the detected operating amount of the brake operating member. For example, the braking effect is represented by a deceleration value of the vehicle. An example of such electrically controlled brake system is disclosed in JP-A 63-20256, wherein the hydraulic pressure in the wheel brake cylinder is controlled to provide a suitable deceleration value of the vehicle which is determined by the electrically detected operating amount of the brake operating member. Described more specifically, the brake system includes (a) a brake operating member, (b) first detecting means for detecting the operating amount of the brake operating member, (c) braking means for applying a brake to a wheel of the vehicle, (d) second detecting means for detecting an actual deceleration value of the vehicle body, and (e) control means for controlling the braking means such that the actual deceleration value of the vehicle body coincides with a target or desired deceleration value which is determined on the basis of the operating amount of the brake operating member.
In the electrically controlled brake system as disclosed in the above publication JP-A 63-20256, the hydraulic pressure in an accumulator is controlled by a solenoid-operated pressure control valve, and the controlled hydraulic pressure is applied to the wheel brake cylinder, to force a brake pad as the friction member against the rotor, so that the wheel rotating with the rotor is braked. The pressure control valve has a pressure-increase position, a pressure-hold position and a pressure-decrease position, for increasing, holding and lowering the pressure applied to the wheel cylinder, respectively. These three positions are suitably selected, with a controlled amount of current applied to a solenoid coil of the valve, so that the hydraulic pressure applied to the wheel cylinder is proportional to the amount of current applied to the solenoid coil. In this brake system, the braking effect is detected in the form of the deceleration value of the vehicle body, and the amount of current applied to the solenoid coil is determined so as to control the hydraulic pressure applied to the wheel cylinder such that the detected actual deceleration value of the vehicle coincides with the target or desired deceleration value determined based on the electrically detected operating amount of the brake operating member.
Another type of electrically controlled brake system is disclosed in JP-A 63-242764, wherein a brake pad which is driven by a motor is forced against the rotor to apply brake to the wheel rotating with the rotor, and the braking effect is detected in the form of a reaction force upon braking of the wheel, more particularly, in the form of an amount of strain of a support member which supports the brake pad and motor, which strain occurs due to a reaction force applied from the rotor to the support member in the circumferential direction of the rotor. The amount of operation of the motor is controlled such that the detected amount of strain or reaction force coincides with a target reaction force determined on the basis of the detected operating amount of the brake operating member.
In the known electrically controlled brake systems as described above, the second detecting means is adapted to detect the deceleration value of the vehicle body such that the deceleration is a decrease in the forward running speed of the vehicle, while on the other hand a decrease in the backward running speed is the acceleration of the vehicle body. Namely, the sign of the electrically detected value of deceleration of the vehicle in the forward and backward running directions is determined to represent the deceleration and acceleration of the vehicle as described above. Although the above arrangement assures suitable control of the braking means when the vehicle is running in the forward direction, the arrangement suffers from an unnecessarily large braking force applied to the wheel, during a backward running of the vehicle, since the decrease in the backward running speed is detected as the acceleration, which means a shortage of the braking force.
The known brake systems also suffer from an unnecessarily large braking force applied to the wheel when the braking means is activated while the vehicle is in a stop state. Described in detail, the actual braking effect in the form of the deceleration value or reaction force indicated above is substantially zero when the vehicle is stopped, and is greatly different from the target deceleration value or target reaction force which is determined by the detected operating amount of the brake operating member. As a result, the amount of braking applied to the wheel is increased so that the actual braking effect coincides with the target value. However, the actual braking effect cannot be increased as long as the vehicle is in the stop state, whereby the control means continues to! increase the amount of braking, resulting in an excessive rise in the braking force applied to the wheel cylinder, brake pad, rotor and other components of the braking means. Consequently, the durability of the brake system is deteriorated.